If you don’t have anything nice to say, management has a tip: Try harder.

Fearing they’ll crush employees’ confidence and erode performance, employers are asking managers to ease up on harsh feedback. “Accentuate the positive” has become a new mantra at workplaces like VMware Inc., Wayfair Inc., and the Boston Consulting Group Inc., where bosses now dole out frequent praise, urge employees to celebrate small victories and focus performance reviews around a particular worker’s strengths—instead of dwelling on why he flubbed...